1. Field of the Invention
This invention relates to a semiconductor laser system, in particular, to an optical system comprised of a frequency-chirped semiconductor laser diode and dissimilar optical paths that are electronically phase-locked.
2. Description of the Background Art
Techniques to control the amplitude and phase of semiconductor laser diodes promise to extend the performance of laser diode systems and thereby enable a wide range of new applications. Presently, semiconductor lasers exhibit several favorable attributes, including low cost, small size and high electrical efficiency, which have made them key elements in communications systems and high power laser systems (e.g., for pumping fiber lasers). One limitation of the latter application, however, is that as the optical power of semiconductor diode lasers and fiber lasers increases, several factors begin to degrade the spectral and spatial quality of the optical output beam. For example, stimulated Brillioun scattering (SBS) can seriously degrade the optical performance of typical implementations of a high power laser and is a major factor limiting the output power of a fiber amplifier.
SBS limits the power of optical amplifiers to <1 KW. Thus, to achieve a high power (>100 KW) laser beam using efficient fiber or solid-state optical amplifiers, one needs to combine the output from multiple amplifiers. Previously, different approaches have been taken to solve the SBS problem. One approach is to suppress SBS by a broadband, phase modulated, or frequency modulated laser (seed) input to increase output power per amplifier to >1 KW; and combine fewer outputs into a high power beam.
For example, in S. Augst, et al., “Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers,” Opt. Lett. 29, 474 (2004), a 25 GHz bandwidth laser seed was employed to suppress SBS (˜100 MHz linewidth) and an acousto-optic frequency shifter (AOFS) was used to phase-lock the path length matched fiber outputs for coherent beam combining Similarly, in G. Goodno, et al., “Active phase and polarization locking of a 1.4 kW fiber amplifier,” Opt. Lett. 35, 1542 (2010), a 25 GHz bandwidth phase-modulated laser seed was used to suppress SBS and an electro-optic phase modulator and a variable delay line were used to phase-lock and equalize path mismatch for coherent beam combining.
U.S. Pat. No. 6,678,294, issued Jan. 13, 2004 and entitled, “Distributed feedback laser apparatus for avoiding stimulated Brillouin scattering,” discloses use of a frequency-modulated DFB seed laser to produce a sawtooth waveform with a period that is no greater than a round trip optical transit time associated with each of the fiber amplifiers to suppress SBS, and phase adjusting means for coherent beam combining.
The main shortcoming of these previous approaches is that the use of a broadband or rapidly modulated seed laser requires strict optical path length matching (<1 mm) of the amplifier paths (>10 to 100 meters) to achieve phase-locking and coherent beam combining. This is normally accomplished by precisely fabricating or cutting optical amplifier path lengths to match and adding mechanical apparatus to eliminate the residual path length errors.
The nonlinearities become more serious for narrow linewidth, single frequency amplifiers suitable for coherent combining Single frequency Ytterbium (Yb), Erbium (Er) and Prasedymium (Pr)-doped fiber lasers and amplifiers utilizing semiconductor diode pump lasers provide limited power (<100's W) before the onset of SBS linewidth broadening, which destroys the mutual coherence of the seed laser and prevents the coherent combination of their outputs.